1. Field of the Invention
The present invention relates to a motor control device that drives and controls a motor.
2. Description of Related Art
In order to perform a vector control of a motor by supplying three-phase AC power to the motor, it is necessary to detect current values of two phases (e.g., U-phase current and V-phase current) among three phases including U-phase, V-phase and W-phase. Although two current sensors (current transformers or the like) are usually used for detecting current values of two phases, the use of two current sensors causes an increase of cost of the entire system equipped with the motor.
For this reason, there is provided a conventional method in which bus current (DC current) between an inverter and a DC power supply is sensed by a single current sensor, and current values of two phases are detected from the sensed bus current. This method is also called a single shunt current detecting method.
FIG. 18 shows a general block diagram of a conventional motor driving system in which the single shunt current detecting method is adopted. An inverter (PWM inverter) 202 is equipped with half bridge circuits for three phases, each of which includes an upper arm and a lower arm, and it converts a DC voltage from a DC power supply 204 into a three-phase AC voltage by switching the individual arms in accordance with a specified three-phase voltage value given by the controller 203. The three-phase AC voltage is supplied to a three-phase permanent-magnet synchronous motor 201, so that the motor 201 is driven and controlled.
A line connecting the individual lower arms in the inverter 202 with the DC power supply 204 is called a bus line 213. The current sensor 205 transmits a signal indicating bus current that flows in the bus line 213 to the controller 203. The controller 203 does sampling of an output signal of the current sensor 205 at appropriate timing so as to detect phase current of a phase in which a voltage level becomes a maximum value (maximum phase) and phase current of a phase in which a voltage level becomes a minimum value (minimum phase), i.e., current values of two phases.
If voltage levels of phases are separated from each other sufficiently, current values of two phases can be detected by the process described above. However, if the maximum phase of voltage and an intermediate phase become close to each other, or if the minimum phase of voltage and the intermediate phase become close to each other, it is difficult to detect current values of two phases. Note that description of the single shunt current detecting method including description of a reason why it becomes difficult to detect current values of two phases will appear later with reference to FIGS. 3, 4 and 5A-5D.
Considering this, in a certain conventional method, if current values of two phases cannot be detected by the single shunt current detecting method in a certain period, current values of three phases are estimated from past current information in the period. More specifically, d-axis current and q-axis current obtained by converting past current values of three phases are inversely converted into current values of three phases so as to estimate them.
In another conventional method, if current values of two phases cannot be detected by the single shunt current detecting method in a certain period, a pulse width of a PWM signal for each arm in the inverter is corrected based on gate signals of three phases in the period.
A usual correction example of a specified voltage value (pulse width) that corresponds to the above-mentioned correction is shown in FIG. 19. In FIG. 19, the horizontal axis indicates time, while 220u, 220v and 220w show voltage levels of the U-phase, the V-phase and the W-phase, respectively. Since a voltage level of each phase follows the specified voltage value (pulse widths) for each phase, they are considered to be equivalent. As shown in FIG. 19, the specified voltage value (pulse width) of each phase is corrected so that “maximum phase and intermediate phase” as well as “minimum phase and intermediate phase” of the voltage do not approach each other closer than a predetermined distance. Thus, voltages of individual phases do not become close to each other to the extent that current values of two phases cannot be detected, and current values of two phases can be detected stably.
However, if such correction of voltage is performed, voltage (applied voltage vector of the motor) becomes discontinuous in a peripheral period in which voltages of individual phases cross each other which may cause a bad influence for smooth drive of the motor.
As described above, although cost reduction of the entire system can be expected by adopting the single shunt current detecting method, there is a problem unique to the single shunt current detecting method. A technique to solve this problem is desired anxiously.